In conventional offshore drilling operations, a top drive is vertically movable in a derrick by means of a drawworks, the drawworks comprising a pulley with a stationary crown block at the top of the derrick and a travelling block connected to and moving together with the top drive. The travelling block and the top drive constitute a travelling assembly. A wire rope runs from a winch through the pulley where it makes several turns around the crown block and travelling block to create a desired mechanical advantage, typically in the range of 16.
The travelling assembly is very heavy, often in the range of 50 to 150 tons, and during drilling operations, it needs to be repeatedly lifted and lowered up and down the derrick. This is an energy-consuming process. Normally, most of the hoisting energy is consumed during tripping, i.e. during the process of pulling the drilling string out of the wellbore and then running it back in. During tripping, it is normally required to lift several hundred tons, often in the range of 500 tons, i.e. the total weight of the travelling assembly and the pipe string. It would be advantageous if the energy consumed in the hoisting loads such as top drives, pipes, pipe strings and pipe stands in offshore drilling operations could be reduced.
Hoisting of heavy loads on drilling rigs has traditionally been done by means of a winch accommodating multiple layers of wire rope. During lifting and lowering, and particularly in heave compensation, the wire rope undergoes numerous bending cycles under load, and is therefore subject to considerable wear. Depending on the number of sheaves in the draw-works, the wire rope on the winch side, the so-called fast line, travels a longer distance than the load, thus requiring multiple layers of wire rope on the winch. Overlying layers of wire rope act with great forces on underlying layers on the winch drum, thus further increasing the wear of the wire rope. Inertia loss in the great number of sheaves in the draw-works also leads to a rather slow acceleration of the load, thus slowing down the operation time. The typical lifetime of a wire rope used together with a multi-layer winch in heave compensation mode on a drilling rig is in the order of two weeks, leading to frequent stops of operation to perform a traditional cut-and-slip to replenish the wire rope.
Prior art hoisting systems on offshore rigs typically use only one wire connected to a winch in one end, running to the top of a derrick through a crown block and down to a travelling block, to which the load is connected, and further to a deadline anchor, typically anchored to the rig floor or to the derrick. When using only one wire, it is of the utmost importance that the wire does not break, as this could cause severe damage to rig and could harm personnel. The fear of wire fatigue also contributes to the frequent replenishment of wire rope. Wires used for heavy lifting operations are very expensive.
Technologies that could increase the lifetime of wire ropes used in hoisting operations, including offshore hoisting operations, and in particular for lifting top drives, drill pipes and stands in active heave compensation and for lifting complete drill strings would be advantageous. Apparatus and systems that improve safety in offshore lifting operations as well as reducing operation times would also be an advance.